1. Technical Field
The present invention relates in general to an improved thermal switch, and in particular to an improved solid state thermal switch for selectively controlling heat transfer to and from a thermoelectric device. Still more particularly, the present invention relates to a system and method for the manufacturing of solid state thermal switches utilizing integrated circuit manufacturing techniques.
2. Description of the Related Art
Conventional cooling systems, such as found in a refrigerator, utilize vapor compression refrigeration cycles to provide heat transfer. Vapor compression cooling requires significant moving hardware, including at a minimum, a compressor, a condenser, an evaporator, and related coolant transfer plumbing. Miniature vapor compression cooling is not available for small cooling applications. However, small cooling applications are highly desirable.
Semiconductors and superconductors have enhanced performance at lower temperatures. CMOS logic can operate materially faster at lower temperatures. For example, if CMOS logic devices are operated at -50.degree. C., their performance is improved by 50 percent over room ambient temperature. Liquid nitrogen cooling of CMOS logic to -196.degree. C. has shown a 200 percent improvement in speed.
Similar benefits have been shown for integrated circuit wiring. Wiring resistances decrease by a factor of two for integrated circuits operated at -50.degree. C. in comparison to room ambient temperature operation.
Thus, sub-ambient temperature operation of integrated circuit logic devices, such as field effect transistors, as well as the interconnect wiring can materially improve integrated circuit performance. However, accomplishing such cooling in the confines of ever decreasing areas poses new challenges.
Thermoelectric cooling is one alternative that has found some utilization given the compact size of Peltier devices. Peltier device thermoelectric cooling is very reliable because such devices are solid state. The utilization of thermoelectric devices in industry has, to date, been restricted to very specialized applications. Due to inefficiencies, very few applications can effectively utilize thermoelectric effects. The undesirable properties of thermoelectric devices, such as high cost and low efficiency, are outweighed by the desirable properties of thermoelectric devices. Recently, there have been significant advances in material technology, many attributable to advances made by the semiconductor industry. The inefficiency of thermoelectric devices is a key negative aspect of implementing a thermoelectric cooling design. A Peltier device cooling system typically has an efficiency in the range of 20 percent for a relatively nominal temperature differential between the hot sink and ambient temperature conditions.
Utilizing a Peltier cooling system to cool at a rate of one watt and attain a sub-ambient temperature of 0.degree. C. requires that the system be powered with five watts. As the amount of heat to be transferred increases, the total power to be dissipated into the ambient mandates large convection devices. Large power supply circuits must also be utilized.
Therefore, Peltier device thermoelectric cooling has not been considered a broadly applicable technology for cooling integrated circuits and improving integrated circuit performance. However, the introduction of an effective solid state thermal switch could boost the efficiency of thermoelectric coolers when utilized in novel configurations as disclosed in cross referenced copending patent applications referred to in the cross reference section of this patent application. The cross-referenced copending patent applications, disclose novel switching for interrupting thermal conduction to and from a Peltier device.
Peltier cooling devices are typically on the order of a few microns in dimension. Micron sized mechanical switches for connecting and disconnecting to thermoelectric devices provides a less than perfect solution. Construction of mechanical micro-miniature thermal switches is not a well developed art. Mechanical micro-miniature switch assemblies would require the manufacture and assembly of contacts, wipers and actuation mechanism which are microscopic. Mechanical micro-miniature thermal switches are costly. Further micro-miniature switch assemblies are unreliable and have short lifetimes.
Generally, moving contacts have a very limited life in comparison to solid state devices. The life of a electro-mechanical switch is measured in cycles. The useful life of a switch might be on the order of a few million cycles. If an electro-mechanical switch must be cycled at a kilohertz, the short lifetime of the switch severely limits practical applications.
The importance of thermal switching of Peltier devices can be explained by classical equations. In operation, a Peltier device transports electrons from a cold source at temperature T.sub.cold to a hot sink at temperature T.sub.hot in response to an electric field placed across the Peltier device. EQU q=.alpha.T.sub.cold I-1/2I.sup.2 R-K.DELTA.T Equation 1
The net heat energy transported by a Peltier device is composed of three elements. In equation 1, the first element represents the Peltier effect (thermoelectric) contribution, the second element defines negative Joule heating or resistive effects, and the third element defines negative conductivity effects of the heat. The thermoelectric component is composed of the Seebeck coefficient, the temperature of operation (T.sub.cold) and the current through the (TE) device.
Approximately one half of the Joule heating produced by the bias current is conducted to the cold source and the remainder to the hot sink. Lastly, the negative element attributable to thermal conduction represents the heat flow or heat conduction through the Peltier device. K is the thermal conductivity of the Peltier device from the hot sink to the cold source. Selective interruption of the heat transfer between a Peltier device and a heat sink has proven superior results as discussed in the copending patent applications referenced above. However, the thermal switch must have low thermal conductivity in the "OFF" state.
In equation 1, the thermoelectric component of the heat transport increases linearly with the current through the Peltier device and the Joule heating increases in proportion to the square of the current. Alternately described, the resistive heating exponentially increases due to the current through the Peltier device while the cooling effect linearly increases with increased current flow. The thermal conduction is also in direct proportion to the temperature differential between the cold source and the hot sink. Equation 1 clearly reflects how quickly a Peltier device in a classical configuration becomes inefficient as the cold source and hot sink diverge in temperature.
Equation 2 below defines a coefficient of performance for a Peltier device. The coefficient of performance is the ratio of the net heat energy transported at low temperature to the power consumed by the Peltier device. For a typical Peltier device made from bismuth telluride material, the coefficient of performance is less than 0.3. ##EQU1##
Note that the numerator of equation 2 represents the net cooling capability of the Peltier device. The denominator of equation 2 represents the total energy provided by an external D.C. power supply. The individual elements of the numerator were described in reference to equation 1. The first element in the denominator is the total Joule heating, while the second element is the heat energy transport work done by the Peltier device in moving energy from the T.sub.cold source to the T.sub.hot sink. Based upon this relationship, the maximum coefficient of performance possible in the configuration of a typical Peltier device is given by equation 3. ##EQU2##
The parameter .gamma. can be expressed in terms of the Seebeck coefficient .alpha., electrical conductivity .sigma. and thermal conductivity .lambda. as set forth in equation 4. ##EQU3##
The first factor in equation 3 T.sub.cold /.DELTA.T is the maximum efficiency possible for any heat pump operating between two thermal sinks T.sub.cold and T.sub.hot. T.sub.cold /.DELTA.T is commonly referred to as the Carnot efficiency. The second factor represents the non-ideal thermoelectric cooling, which can also be characterized by a figure of merit ZT. As .eta..fwdarw.(T.sub.cold /.DELTA.T) as .gamma..fwdarw..infin.. To date it has been very difficult to develop a thermoelectric material in a configuration which yields high values of ZT.
Another constraint of Peltier device cooling is that only a limited temperature excursion below ambient temperature is attainable. The temperature differential limitation arises from the fact that the effective temperature span is constrained by efficiency. Efficiency of a thermoelectric device degrades quickly with an increasing temperature differential between a hot sink and a cold source. The maximum temperature differential possible T.sub.max is given by equation 5 below. EQU .DELTA.T.sub.max =1/2ZT.sub.cold.sup.2 Equation 5
For bismuth telluride having a ZT of approximately 0.3, T.sub.max is 45.degree. K. at 300.degree. K., where 32.degree. f. is equivalent to 273 K.
Thus, there are a number of very fundamental constraints on efficiency and differential temperature that limit the practical utilization of conventional static thermoelectric cooling applications. Particularly, statically coupled applications which utilize ambient temperatures to dissipate the heat are impracticable.
Equation 1 is the classic equation for static operation of a Peltier device. However, equation 1 does not apply when thermal and electrical switching are introduced to create a non-linear system. Many inefficiencies of equation 1 can be avoided if an effective thermal switch is implemented.
Typically, each Peltier device is small in dimension and can only transport a finite amount of heat. Therefore, to produce a cooling effect of desired magnitude many Peltier devices must be connected together. Selectively thermally coupling of Peltier devices to heat sources or sinks utilizing thermal switches greatly increase the efficiency of operation and the inherent differential temperature limitation.
Thermal conductivity within a semiconductor occurs from mobile carriers and phonon conductivity. Phonon conductivity in a lattice structure is due to changes in force which atomic planes exert on neighboring planes. The lattice force is due to vibrations of the lattice structure about normal lattice sites. Thermal transfer due to phonon conductivity is relatively unrelated to current flow. Phonon conduction is the same mechanism by which sound is transmitted through a crystal structure.
Thus, the mechanism of Peltier heating and cooling is one of heat storage or release by mobile-carrier populations, and the Peltier coefficient is the energy carried per unit charge. During operation, carriers are injected into a region where their energy is significantly different from the average thermal energy of the normal carrier population.
Due to their substantial numbers and energy difference, injected carriers change the average energy of a region and therefore, the region which the carriers enter changes in temperature. A temperature difference between two surfaces bounding a material usually results in the flow of thermal energy from the hotter area to a cooler area.
Transistors, including metallic oxide semiconductor field effect transistors (MOSFETS), can be utilized as lossy thermal switches. When the gate voltage of a MOSFET is below its threshold voltage, the MOSFET is "OFF". When the MOSFET is "OFF" there is minimal current flow. Hence, the thermal conduction due to the flow of electrons is negligible. However, even in the OFF state, thermal conduction across the MOSFET due to lattice conductivity is significant and cannot be controlled by the MOSFET. Thus, MOSFET thermal switches are lossy and provide poor thermal isolation.
It should therefore be apparent that there is a need for a solid state thermal switch to provide intermittent thermal coupling to thermoelectric devices. Further, a miniature solid state thermal switch which has low thermal conductance in the OFF state and a high thermal conductance in the ON state would be highly desirable. Additionally, a thermal switch which can be produced utilizing integrated circuit manufacturing techniques is in demand.